Thermionic cathode



Patented Oct. 25, 1932 e GEORGE P. HALLIWELL, OF WILKINSIB'UBG, PENNSYLVANIA, ASSIGNOR TO WESTING- HOUSE ELECTRIC & MANUFACTURING COMPANY,.A CORPORATION OF PENNSYL- VAN IA THERMIONIC CATHODE No Drawing.

- capable of emitting electrons at temperatures lower than cathodes composed of oxidecoated platinum or platinum alloys.

Another object of my invention is to provide an improved alloy or aggregate for the cathodes of space discharge devices that has a comparatively high specific resistance, that may be made from cheap and readily obtainable materials and that has physical properties which are superior to oxide-coated 29 filaments.

Another object of my invention is to provide improved vacuum tubes or space discharge devices, and especially those of the three or four electrode type having output and control electrodes, and a cathode comprising an alloy or aggregate that is capable of emitting electrons when heated to a moderately high temperature.

A further object of my invention is to pro- 30 vide a method of producing an alloy or aggregate in which one or more of the alkaline earth metal oxides are incorporated that may be forged and drawn into wires or filaments capable of being utilized as the cathode in space discharge devices.

It has heretofore been the practice to employ tungsten, thoriated tungsten and oxidecoated filaments of platinum or platinum alloys as the cathode in space discharge devices, such as X-ray tubes, hot cathode rectifiers and vacuum tubes of the three and four electrode type that are utilized for radio purposes.

Vacuum tubes having cathodes made of tungsten or thoriated tungsten have a comparatively short life and the electron emission, is somewhat irregular. Oxide coated filaments having a foundation metal of platinum or nickel, or alloys of these metals, have been .in extensive use.' Oxide coated fila- 1' Application filed December 14, 1928. Serial No. 326,155.

ments, however, are not altogether satisfactory because of the difiiculty of producing filaments of this type having uniform physical properties.

In producing oxide coated cathodes inaccordance with the present practice, it is difficult to obtain a filament having an adherent layer of the oxide or one which has a uniform thickness of the oxide coating on the wire. There is also considerable difliculty $0 in-obtaining true temperature measurements of oxide-coated filaments to determine the temperature at which the tube is to be operated because they are formed of two lay-' ers; namely, the foundation metal and the superimposed oxide coating. Oxide-coated filaments also have a rough soft surface and care must, therefore, be exercised in handling the material to prevent the oxide coating from being removed or contaminated 7 with oil or grease while it is being mounted in the tube.

The mechanism of thermionic emission from oxide coated surfaces is not clearly understood, but according to the latest theory, part of the oxide is reduced to the metallic state in minute quantities which forms a superficial filament of high thermionic activity. Only a layer of metal one molecule thick is presumed to give satisfactory emis-- s1on.

In making experiments in accordance with the above specified theory, I have made the discovery that when rn alkaline-earth-metal oxide is disseminated through or alloyed I with a metal which. is suitable as a base for electron emitting bodies, such as nickel, to i which is added a metal which is highlg active toward oxygen, such as titanium, a lament material is provided that not only gives good emission, but is free from the disadvantages resulting from the use of oxide-coated material. Y

I'n practicing my invention, I may form intimately-powdered mixtures of a foundation metal, such as nickel with a metal which is highly active toward oxygen, such as titanium, and the oxides of one or more of the alkaline earth metals, such as calcium, barium or strontium, or I may mix powaeaaaes a such a composition have good hy'sical properties and also have the advan a of giving electron emission at aboutlO .lower temperature than platinum. A small percentage of other metals which are highly active toward oxygen, such as thorium vanadium, tantalium, silicon, manganese, a uminum, or metals of the rare earth group such as cerium 0r zirconium, or mixtures thereof, may be m snbstituted in whole or in part for the vtitamum. a Other modifications of my invention will become apparent to those skilled in the art,

and I'desire, therefore, that only'such limita- 5 tions shall be placed on my invention as are imposed by the prior artand the appended claims. 7 p

-I claim as my'in'vention: I

Y 1. Electron emitting material of substantially homogeneous character capable of be ing forged .into wire consisting of a base metal selected from a group consisting of nickel and cobalt, said base metal containing 1 about 0.5 per cent titanium and about 10 per centlmetal selected from a group consisting of titanium and iron, with about 2 to 10 per cent alkali-earth metal oxide uniformlydistributed therethrough. 2. Electron emitting materialof substan- 80 tially homogeneous character capable of being forged into wire consisting of a base metal of about/10pm cent nickel, per cent cobalt, 0.5 per centtitanium and the balance. selected from a group consisting of iron and 85 titanium, with about 2 to 10 per cent alkaliearth metal (oxide uniformly disseminated therethrough. v

3. Electron emitting material of substantially homogeneous character capableof beso ,ingforged into wire consisting of a base metal of about per cent nickel, 20 per cent cobalt, 8 per cent iron and 2; per cent titanium with about 4 per cent alkali-earth metal oxide uniformly distributed therethrough." as In testimony whereof, I have hereunto subscribed my name this 12th day of December 1928. GEORGE-P. 

